As shown at Charles Leski's latest Coin, Banknotes, Stamps and Postal History auction (now held under the Mossgreen banner) an Australian banknote normally valued at $5 can be worth over $1000 if someone has accidentally stuffed it up.

Missing: Serial number and 'Australia' are there, but not much else. Sold for $701.50.

This 1993 polymer note has a nominal value of $10 but because it is missing the intaglio printing phase on the front — the final stage in the printing process, that adds the fine details, such as writing or complex artwork – it's now worth much more. Adding to its value, there's no serial number on the reverse and the checker who spotted the error has added their ID number in felt pen.

Sold in "uncirculated" condition, it fetched $1220 IBP at the September 30 sale in Melbourne.

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A 1992 $5 polymer note, also missing the intaglio phase, serial number and signature of the then-governor of the Reserve Bank, Bernie Fraser, also sold for $1220. "It's the super of the future" as Bernie would say. Other examples of error notes sold for $701, $671 and $732, all IBP.

Lot 197, a rare set of three $5 error notes with consecutive serial numbers, again uncirculated, was listed for $3000 to $4000 but was passed in.

Real thing: This is what you should have seen - Banjo Patterson, wild horses and the number 10

Leski suspects that he picked these up as part of a larger collection of notes but because had no personal interest in them he decided to sell them. Chances are he was happy he did. He made a lazy $5000 or so.

The question is, how do such obvious duds make it into the public domain?

Leski says that even in the polymer period the quality control of banknotes was initially done by humans, so the occasional misprint passed through the system and escaped into the real world. Among enthusiasts, the magnitude of the error is what determines value at auction. Errors can range from a missing serial number to, well, what you see here.

Condition is also important and these examples were obviously spotted early in their life-span and handed over to someone who cared. Twenty years later and, bingo, their face value has jumped by more than a thousand dollars in some cases.

This is still a relatively new niche in the auction market so a cautious Charles Leski says he can't be sure if they will ever take off in a big way.

"The majority of collectors want one of every note so this area represents something much more exotic," he says. "It's really into the rarefied, more expensive end of the market." It's too early to say whether they represent a good investment or not.

"The main point," says Leski, "is that when you are ready to sell it's important to find another like-minded soul who is wanting to buy." You only need one. Two is a bonus.

Graham Shepherd is a pioneer in this specialist area.

A long-time collector and dealer in rare banknotes, he started to concentrate on error notes in 1988 and has since discovered that others share this eccentricity. He estimates there are a few hundred like-minded souls.

According to Shepherd the ones worth collecting are the bleedingly obvious, as in notes missing one colour or totally blank on one side. Examples of the latter also turn up on occasion.

Needless to say, if one appears in your spare change, keep it. What about counterfeit notes? Shepherd says these are also of interest to some collectors, despite the grey legality of owning them.

"I won't say whether I have any or not," he says, "but yes, they are quite collectable."